I faced the same problem on one of my brass cabooses. The bolster on this
caboose is composed of a turned brass center post with a pair of
fabricated/decorative brass bolster side pieces. The turned bolster center
post was screwed to the bottom of the floor from inside. My solution was to
first remove and discard the fabricated bolster side pieces (you don't want
them touching the center post and you can't see them anyway). I then
redrilled the screw hole for the center post so that it was larger that the
diameter of the bolster center post by about a 1/16". A piece of .040"
styrene about 3/4" square was mounted inside the caboose body over this
enlarged hole and the bolster center post attached to the styrene with a
screw from the top/inside. The result is that the bolster center post is no
longer touching the brass floor and is isolated. A small brass solder tab
was positioned under the screw holding the bolster center post to the
styrene...the wire to the lighting circuit attaches to this tab thus making
a connection to the truck and one rail. Because the bolster center post is
now screwed to the styrene rather than the floor, that end of the caboose
will ride lower by the thickness of the floor but that difference is so
small I ignored it. If it is a problem, you could insert a styrene washer
equal to the thickness of the floor between the top of the bolster center
post and the styrene.
The only critical step is to align the bolster center post in the enlarged
hole so that it doesn't touch the brass floor. I did this by screwing the
bolster center post to the styrene (with the solder tab in place) and then
placing the piece of styrene on the floor with the center post in the
enlarged hole. I clamped the styrene in position and then turned the floor
over to check that the center post was correctly aligned with the hole. Once
everything was correct, I bonded the styrene to the floor with CA. I think I
reinforced this bond with a pair of screws up into the styrene from below
the floor along the centerline of the car.
There are probably easier ways to solve this problem but this was the best I
could come up with...